[Bug tree-optimization/117043] New: missed vectorization opportunity: data[i] = data[i - a[i]] + 1; (a[i]=0)

[652023330028 at smail dot nju.edu.cn via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117043

            Bug ID: 117043
           Summary: missed vectorization opportunity: data[i] = data[i -
                    a[i]] + 1; (a[i]=0)
           Product: gcc
           Version: 15.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: 652023330028 at smail dot nju.edu.cn
  Target Milestone: ---

Hello, we noticed that there seems to be a missing vectorization for the code
below.

reduced code: https://godbolt.org/z/zxKhKWxGq

int data[100];
void f(int a[100])
{
    for(int i = 0; i < 100; i++){
        a[i] = 0;
    }
    for(int i = 0; i < 100; i++){
        data[i] = data[i - a[i]] + 1;
    }
}

GCC -O3 -fno-vect-cost-model:
f(int*):
        mov     QWORD PTR [rdi], 0
        mov     rsi, rdi
        lea     rdi, [rdi+8]
        xor     eax, eax
        mov     QWORD PTR [rdi+384], 0
        mov     rcx, rsi
        and     rdi, -8
        sub     rcx, rdi
        add     ecx, 400
        shr     ecx, 3
        rep stosq
.L2:
        mov     edx, eax
        sub     edx, DWORD PTR [rsi+rax*4]
        movsx   rdx, edx
        mov     edx, DWORD PTR data[0+rdx*4]
        add     edx, 1
        mov     DWORD PTR data[0+rax*4], edx
        add     rax, 1
        cmp     rax, 100
        jne     .L2
        ret

<bb 3> [local count: 1063004408]:
  # ivtmp.6_33 = PHI <ivtmp.6_37(3), 0(2)>
  # DEBUG i => NULL
  i_26 = (int) ivtmp.6_33;
  # DEBUG i => i_26
  # DEBUG BEGIN_STMT
  _7 = MEM[(int *)a_16(D) + ivtmp.6_33 * 4];
  _8 = i_26 - _7;
  _9 = data[_8];
  _10 = _9 + 1;
  MEM[(int *)&data + ivtmp.6_33 * 4] = _10;



Expected code: vectorize the loop body.
For example:
<bb 3> [local count: 265751102]:
  # ivtmp.14_19 = PHI <ivtmp.14_21(3), ivtmp.14_4(2)>
  # DEBUG BEGIN_STMT
  _5 = (void *) ivtmp.14_19;
  vect__4.7_22 = MEM <vector(4) int> [(int *)_5];
  vect__5.8_20 = vect__4.7_22 + { 1, 1, 1, 1 };
  MEM <vector(4) int> [(int *)_5] = vect__5.8_20;
  # DEBUG BEGIN_STMT
  # DEBUG BEGIN_STMT
  ivtmp.14_21 = ivtmp.14_19 + 16;
  if (_11 != ivtmp.14_21)
    goto <bb 3>; [95.96%]
  else
    goto <bb 4>; [4.04%]

Thank you very much for your time and effort! We look forward to hearing from
you.